Obtaining of pyrogenic silica is accomplished, according to known processes, by reaction of volatile compounds of silicon in vapor form at elevated temperatures in the presence of hydrolyzing and possibly also oxidizing substances or mixtures of substances. The production takes place, for example, such that the volatile compounds to be reacted, especially halides of silicon in the gaseous phase, are exposed to the hydrolyzing influence of water vapor (steam) or of these producing gas mixtures, whereby the developing oxide is formed as an aerosol and subsequently is isolated at temperatures above the dew point of the gaseous reaction product, which may be condensed easily. In this case the steam-forming gas mixtures may consist of combustible, particularly hydrogen-containing gas or gases forming said hydrogen, and of non-combustible, perferably oxygen containing gases. Of the compounds to be considered as starting substances, the use of volatile halides, especially of chlorides or fluorides, is particularly advantageous.
For purposes of the present invention, any pyrogenic silica may be used that is capable of being treated and converted into a hydrophobic silica. Various methods are known in the art for producing pyrogenic silica; see for instance U.S. Pat. Nos. 2,871,140; 2,876,119; 2,882,254; 2,892,730; 2,898,391; 2,957,044; 2,990,245; 3,006,738; 3,033,801; 3,083,115; 3,086,851; 3,103,495.
For many purposes it is desirable to impart organophilic or hydrophobic characteristics to fillers, especially for incorporation of highly dispersed fillers into organic media. Thus it has been known to render powdery silica hydrophobic by treatment with suitable organic silicon compounds, for example, alkylchlorosilanes. According to this procedure, the chlorosilane is converted with water and adsorbed on the surface of the silica while forming hydrochloric acid. The hydrophobic silica produced thereby must be subsequently freed from the hydrochloric acid. Representative organic silicon compounds that may be used are dimethyldichlorosilane, ethyltrichlorosilane, amyltrichlorosilane, vinyltrichlorosilane, phenyltrichlorosilane, methyltrichlorosilane, methyldichlorosilane, methylvinyldichlorosilane, trimethylchlorosilane, diphenyldichlorosilane, bis-trichlorosilylethane and bis-trichlorosilylbenzol.
The process of imparting hydrophobic properties to the silica may also be accomplished in the case of powdery silica by treatment with silicon oil to form a coating. For this, the dry powdery, silica must be suspended in an organic liquid. Suitable silicon oils are linear organopolysiloxanes. The free valences not bound to oxygen of the silicon atom may be satisfied by organic groups such as alkyl, aryl or hydrogen. Examples are methyl and C.sub.6 H.sub.5. The viscosity of these silicon oils is generally between 3 and 1000 cp at 20.degree. C. A specific example of this material is Tegiloxan R 50 which is understood to be a linear dimethylpolysiloxane with a viscosity of 50 cp.
A pyrogenic silica which had been rendered hydrophobic according to the process described in German Pat. No. 11 63 784, corresponding to U.S. Pat. No. 3,924,029 which is relied on and incorporated herein by reference, has proven itself particularly advantageous for use in shoe polish. It should be noted that silica which has been made by a pyrogenic process and treated to render it hydrophobic by processes other than that shown in U.S. Pat. No. 3,924,029 may also be incorporated into shoe polish compositions according to this invention.
According to the process described in the German Pat. No. 11 63 784 and U.S. Pat. No. 3,924,029, pyrogenic silica is subjected to a surface treatment by treatment with an organohalosilane whereby the pyrogenic silica is kept in a whirling suspension. And, after proceeding as far as possible with the liberation of halogen, hydrogen halide and adsorbtively bound water is mixed with organohalosilanes in the absence of oxygen and as homogeneously as possible. This mixture, together with small quantities of steam and optionally together with an inert gas, is continuously fed to an upright, pipe shaped oven forming the processing chamber and is heated to a temperature in the range of about 200.degree. to 800.degree. C., preferably 400.degree. to 600.degree. C. The solid and gaseous reaction products are separated and the solid product may be optionally secondarily de-acidified and dryed, whereby any contact with oxygen prior to cooling down to below 200.degree. C. is to be avoided.